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698          "                  Forced Draught.

Again, we have per Ib. of such fuel '835 Ib. of C and '042 Ib.
of H,

Heat units.

.-. -835 Ib. Carbon + 0      gives  14,500 x -835 = 12,107
•042 Ib. Hydrogen 4- *O gives 62,032 x '042 =   2,605

Total units            .    14,712

By careful laboratory experiment one Ib. of such coal is found
to have a calorific value of 14,701 thermal units, and evaporate
15 Ibs. of water at 212°. Also 12 Ibs. of air are required per Ib.
of fuel. (See pp. 906 and 1148.)

In actual practice considerably less beat is developed, and the
evaporation is good at 10 Ibs. of water, being commonly 6 or 8.
Also 24 Ibs. or 312 cub. ft of air are required, with natural
draught^ to dilute the gases, and allow the air to reach the fuel.

Forced • Draught.—The essential advantage of forced
draught lies in the fact that a smaller dilution of the gases can
be allowed, i81bs. of air per Ib. of fuel, or only i|- times what
the chemist requires. In consequence, a higher temperature is
obtained, the grate and heating surface being much more efficient;
and thus a smaller boiler will serve the purpose, a great advantage
in torpedo boats.

The air must not be solely fed through the fire bars, or a
tongue of flame would meet the stoker whenever he opened the
fire door. The closed stokehold, the earlier method of solution,
places the stoker in a plenum of air at a moderate pressure, which
enters the furnace as usual. The later method, the closed ash-
pit, requires a box-shaped fire door, into which air is fed as
well as to the ashpit; but the air to the latter is at a much lower
pressure. The air from the box door passes to the coals through
holes in the baffle-plate, and the supply is cut off automatically
whenever the door is opened. Both methods still have their
advocates. The pressure is caused by a fan. (See App* 12.,p. 907.)

Waste of Fuel is largely due to formation of smoke and
incomplete combustion, the carbon partly being burnt to CO.
Alternate or continuous firing, by careful men or mechanical
stokers, and a sufficient supply of air, are the only remedies.
The gases also pass up the chimney at a greater heat than 600°,